Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 348 AM EDT Sun Jul 18 2021 Valid 12Z Sun Jul 18 2021 - 12Z Tue Jul 20 2021 ...Heavy rain and scattered flash flooding possible throughout the Southwest, Lower Mississippi Valley, and southern New England to end the weekend... ...Excessive heat continues across the Northern Plains and Northern Great Basin... ...Threat of dry lightning over central and northern California today could spark new wildfires... An area of low pressure centered over southern New England and associated slow moving cold front is expected to remain the focus for scattered thunderstorms and heavy rain through early this week. A few downpours could lead to flooding concerns for parts of southern New England today, where copious amounts of rain have fallen so far this month. Farther south, repeating rounds of thunderstorms are expected along the aforementioned cold front between the Southeast and Southern Plains, with the boundary becoming stationary at times. Due to ample moisture in place, a few thunderstorms could produce intense rainfall rates and scattered flash floods. A Slight Risk (level 2/4) of Excessive Rainfall is in effect today for portions of the Lower Mississippi Valley, with a large Marginal Risk (level 1/4) covering much of the southeastern and south-central U.S. between the Carolinas and Oklahoma. The heavy rainfall threat is forecast to inch southward on Monday, with the greatest flash flood chances found along coastal sections of the Southeast, as well as northern Louisiana and eastern Texas. Meanwhile, the summer monsoon is in full swing across the Southwest, which means more chances for thunderstorms and excessive rainfall today. Sections of central and southeast Arizona, as well as much of New Mexico could see isolated rainfall amounts up to an inch. Flash Flood Watches are in effect across central New Mexico. The dog days of summer are set to continue across the Northern Plains and Northern Great Basin. High temperatures well into the triple digits are forecast today and Monday for eastern Montana, where a few daily high temperature records could be tied or broken. Another concern with the ongoing heat wave is that low temperatures are forecast to only drop into 70s at night, making it harder to recover from the heat after sunset. Highs above the century mark are also expected for the lower elevations of the Intermountain West. Excessive Heat Warnings and Heat Advisories have been issued. Residents are advised to drink plenty of fluids and avoid extended time outdoors in order to avoid heat-related illness. Monsoonal moisture entering parts of California and the Sierra Nevada today may fuel isolated showers and thunderstorms. While rain is desperately needed across this portion of the country, much of these thunderstorms are expected to remain dry. The main threat will be associated with lighting and the potential for new wildfires. Any lightning strikes reaching the ground could easily ignite the dehydrated terrain throughout the region. Fire Weather Watches and Red Flag Warnings have been issued in order to highlight the threat. Snell Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php